Self-healing polymeric materials have gained substantial importance in recent times due to its inherent ability to repair damages caused by mechanical deformations, corrosion by chemicals or degradation in adverse atmospheric conditions thus preventing catastrophic failure and increasing the life time of materials. See, e.g., E. B. Murphy, F. Wudl, Progress in Polymer Science 2010, 35, 223; C. C. Corten, M. W. Urban, Advanced Materials 2009, 21, 5011; M. W. Urban, Progress in Polymer Science 2009, 34, 679; S. D. Bergman, F. Wudl, Journal of Materials Chemistry 2008, 18, 41; and D. Y. Wu, S. Meure, D. Solomon, Progress in Polymer Science 2008, 33, 479. These adverse repetitive external stimuli result in formation of micro cracks or cavities eventually leading to macroscopic damage. Crack healing has been accomplished by wielding, incorporating hollow fibers or micro encapsulation of healing monomers and also by employing thermally reversible Diels-Alder reaction. See, e.g., J. W. C. Pang, I. P. Bond, Composites Science and Technology 2005, 65, 1791; J. W. C. Pang, I. P. Bond, Composites Part A 2005, 36, 183; J. M. Kamphaus, J. D. Rule, J. S. Moore, N. R. Sottos, S. R. White, Journal of the Royal Society Interface 2008, 5, 95; R. J. Cardoso, A. Shukla, Journal of Materials Science 2002, 37, 603; E. N. Brown, M. R. Kessler, N. R. Sottos, S. R. White, Journal of Microencapsulation 2003, 20, 719; X. Chen, F. Wudl, A. K. Mal, H. Shen, S. R. Nutt, Macromolecules, 2003, 36, 1802; Q. Tian, Y. C. Yuan, M. Z. Rong, M. Q. Zhang, Journal of Materials Chemistry 2009, 19, 1289; and Y. Zhang, A. A. Broekhuis, F. Picchioni, Macromolecules 2009, 42, 1906. However, reversible photocyclizations are of great commercial importance due to their capability to heal repeated damage at the same position. Moreover, these photo-reactions are inexpensive and environmentally friendly alternative to harsh reagents in order to enact chemical transformations.
Olefin compounds such as cinnamic acid, anthracene, thymine, and coumarin undergo [2+2] cyclodimerization upon irradiation with ultraviolet (UV) light of wavelength (λ)>300 nm to form cyclobutane, which revert back to starting olefins upon irradiation with shorter wavelength of light. M. Hasegawa, Y. Suzuki, N. Kita, Chemistry Letters 1972, 317; A. Reiser, Photoreactive Polymers: The Science and Technology of Resists; Wiley-Interscience: New York, 1989; Y. Zheng, M. Micic; S. V. Mello, M. Mabrouki, F. M. Andreopoulos, V. Konka, S. M. Pham, R. M. Leblanc, Macromolecules 2002, 35, 5228; Y. Chen, J. L. Geh, Polymer 1996, 37, 4473. Chung et al. had utilized functional cinnamoyl polymers for photochemical self-healing. These polymeric networks cross-linked via cycloaddition to form cyclobutane groups, these highly strained dimers breaks preferentially upon crack propagation which can be restored by using appropriate wavelength of light. C. M. Chung, Y. S. Roh, S. Y. Cho, J. G. Kim, Chemistry of Materials 2004, 16, 3982. Coumarin derivatives and analogues contain β-ketoester which are well-known for their photochemical properties, undergoes a reversible photoinduced cycloaddition upon irradiation with light λ>310-355 nm forming dimers, joined by cyclobutane ring whereas the reverse photo cleavage reaction occurs at shorter λ<260 nm. N. Ramasubbu, T. N. G. Row, K. Venkatesan, V. Ramamurthy, C. N. R. Rao, Journal of the Chemical Society Chemical Communications 1982, 3, 178; Y. Chujo, K. Sada, T. Saegusa, Macromolecules 1990, 23, 2693; V. Ramamurthy, K. Venkatesan, Chemical Reviews 1987, 87, 433; L. H. Leenders, E. Schouteden, S. F. C. De, Journal of Organic Chemistry 1973, 38, 957. Coumarins are well known and used in the field of biology, medicine and polymer science. They find widespread applications as liquid crystalline polymers, photoactive surface, polymeric electroluminescence and biomaterials. Chen and co-workers synthesized a wide variety of coumarin functional polyesters, polyethers and polyurethanes and studied there reversible chain extension as well as chain scission. Y. Chen, R. T. Hong, Journal of Polymer Science, Part A: Polymer Chemistry 1997, 35, 2999; Y. Chen, C. S. Jean, Journal of Applied Polymer Science 1997, 64, 1759; Y. Chen, K. H. Chen, Journal of Polymer Science, Part A: Polymer Chemistry 35, 613; Y. Chen, C. S. Jean, Journal of Applied Polymer Science 1997, 64, 1749. Trenor and others have studied the photo-reversible reactions of polyethylene glycol functional coumarin. S. R. Trenor, T. E. Long, B. J. Love, Macromolecular Chemistry and Physics 2004, 205, 715. The photo reaction kinetics of coumarin functional polymers has also been extensively investigated with emphasis on the rate of formation of dimers in presence and absence of substituents in coumarin and its analogs.
Organic light emitting devices (OLED), photovoltaic (PV) and solar cells are susceptible to damage upon exposure to hot and humid environmental conditions. J. Gaume, P. Wong-Wah-Chung, A. Rivaton, S. Sandrine The'rias, J. Gardette, RSC Advances, 2011, 1, 1471: P. R. Elowe, M. A. Stempki, S. J. Rozeveld, M. W. DeGroot, Chemistry of Materials 2011, 23, 3915. This reduces the active lifetime of the devices. Rigid or flexible organic and inorganic coatings such as polymethyl methacrylate (PMMA) or PMMA-polyolefin are used to support and provide protection to the underlying semiconductor layers. K. Todd, H. Kedar, U.S. Patent Application Publication No. US 2011/0315206 A1, published Dec. 29, 2011; A. Jochen, S. Florian, International Patent Application Publication No. WO 2012/010360 A1, published Jan. 26, 2012; E. A. Gun, D. G. Theodore, S. B. Joseph, U.S. Patent Application Publication No. US 2011/086183 A1, published Apr. 14, 2011. These coatings however, may themselves be susceptible to formation of cracks or micro scratches upon prolonged exposure to unfavorable environmental conditions. These micro scratches act as stress concentrators sites which compromise moisture barrier properties. There is therefore a need for encapsulation materials that exhibit self-healing, in addition to low permeability to water and oxygen, optical transparency, and flexibility.